The present invention relates to a modulator of the Mach-Zehnder type.
The invention lies in the field of optoelectronic systems used for the transmission or optical processing of optical digital data.
In these systems, the information is often in the form of binary data represented by pulses modulating an optical carrier wave supplied by a laser. In the amplitude domain, the quality of an optical signal is normally defined by two parameters: the signal to noise ratio and the extinction ratio. The signal to noise ratio is defined as the ratio of the optical power of the signal to the noise power in a wavelength band containing the wavelength of the signal carrier. The extinction ratio is defined as the ratio of the powers corresponding respectively to the high and low levels of the signal.
There exist more particularly two coding techniques for the high-output sources. A first coding technique consists in producing a modulator by electro-absorption. Such a modulator is very simple in principle. It is composed in fact of a guide to which a control voltage is applied. This voltage moves the absorption peak of the semiconductor structure of the guide towards the working wavelength. The absorption profile in the guide therefore depends on the control voltage, which makes it possible to produce an amplitude modulation of the wave passing through the guide. Nevertheless, this coding technique then gives rise to frequency modulation problems related to the amplitude modulation.
A second technique of coding an optical wave consists in using a modulator of the Mach-Zehnder type. Such a modulator is more difficult to implement but makes it possible to effect better control of the frequency modulation.
A modulator of the Mach-Zehnder type is generally produced in a semiconductor substrate in order to facilitate its integration with other active components. Preferentially, the Mach-Zehnder is produced on an InP or AsGa support or on any other III-V material. The Mach-Zehnder can however be produced in any type of semiconductor material. More particularly, a Mach-Zehnder modulator is in the form of an active monolithic structure with uniform amplification over the entire substrate and sections which are electrically insulated in order to form the branches.
A modulator of the Mach-Zehnder type according to the state of the art is illustrated in FIG. 1. Such a modulator functions essentially as an interferometer. This is because a Mach-Zehnder modulator is composed of two branches conveying two coherent waves coupled in order to form the output signal. A light beam is separated in two by means of a semi-reflecting separator plate or by means of an electro-optical coupler.
An electrode is placed on each branch in order to modulate the phase by an electro-optical effect. The electro-optical effect results from an interaction between the guided optical wave and an electrical field which causes a modification of the permittivity of the environment. This interaction makes it possible to modulate the phase or amplitude of the optical wave. At the output of the branches, the light signals are collected by a second semi-reflecting plate or by a second coupler in order to form interferences.
Transmission will be maximum in the case of constructive interference, when the two branches are in phase, and minimum in the case of destructive interference when the branches are in phase opposition. Such a modulator makes it possible to effect an analogous modulation of the light intensity.
More particularly, the branches are composed of a medium whose index varies according to the optical power conveyed. A control voltage is applied to one branch, referred to as the active branch, and causes a change in index in this branch in order to modulate the optical signal passing through the active branch and thus obtain constructive and destructive interferences at the point of recombination of the two branches.
In contradistinction, the branch opposite the active branch is referred to as the passive branch.
One particular problem results however from this technique: the change in index in the active branch of the Mach-Zehnder also gives rise to a variation in absorption which causes a modification in the amplitude of the output power of this branch. In particular, the output power of the active branch will not be identical to the output power of the passive branch. The interferences will therefore not be totally destructive. The output powers not being equal at the recombination point, the interference will not be total and the coding will be impaired.
It has been established that the variation in absorption depends on the wavelength of the optical signal and has a peak for a given wavelength. The Mach-Zehnder modulator will therefore in general be used for a working wavelength distant from the absorption peak.
Nevertheless, the variation in absorption is never completely zero at the working wavelength, and consequently the interferences are never completely destructive.
The solutions offered by the prior art consist mainly in optimising the composition of the semiconductor layers which make up the structure of the Mach-Zehnder in order to optimise the variation in the index ratio for a given absorption. The solutions of the prior art have however reached a physical limit and the problem is not resolved yet.